1. Field of the Invention
The present invention relates to methods of manufacturing light diffusion film, a light diffusion film, a polarizing plate, and a liquid crystal display.
2. Description of the Related Art
Recently, liquid crystal displays (LCDs) are used widely in, for example, PCs, automated teller machines (ATMs), and car navigation system equipment. Among LCDs, in a transmissive LCD with a twisted nematic (TN) mode employed therein, for example, viewing angle dependency is high, picture quality changes considerably when it is viewed from an oblique direction at a certain angle or more, and an image to be displayed in black under normal conditions appears whitish, and thus the contrast ratio may decrease or tone reversal may cause difficulty in viewing normal display. Therefore, a method has been proposed in which a light diffusion film is disposed on a front surface of a device for the purpose of securing the viewing angle of a LCD (for example, see JP 10 (1998)-10513 A). According to this proposal, the tone reversal is suppressed and the viewing angle increases but there is a problem in that the contrast in the front direction decreases. Therefore, a light scattering film has been proposed that adjusts inner refractive-index distribution in order to cancel the decrease in image quality while balancing a tone reversal prevention effect with contrast maintenance (for example, see JP2006-133463 A and JP2007-271673 A). Such light scattering film is obtained by forming interference fringes derived from an original light diffusion film inside of a resin film by irradiating light excellent in coherent such as a laser beam onto a resin film made of photosensitive resins having different refractive index through the original light scattering film. Since the interference fringes formed in this manner show properties of the original light diffusion film, the resin film thus obtained is a light diffusion film. According to this method, a resin film having various light diffusion properties can be obtained by controlling the light diffusion properties of the original light diffusion film.
However, as schematically shown in FIG. 2, a laser beam that has been incident from the side of an original light diffusion film 13 is reflected at an interface between air and a light transmissive substrate 21 that is formed on the other side of a photosensitive resin layer 12. The incident laser beam (O-order light 14, diffusion light (primary light) 15) may be interfered with specular reflection light 26 at the rear surface, and thereby a reflection hologram may be formed. A schematic view of an example of a transmission hologram structure, in which interference fringes derived from an original light diffusion film are formed inside of a resin film, is shown in FIG. 4. A schematic view of an example of a reflection hologram structure formed by interference between incident light and specular reflection light at the rear surface is shown in FIG. 5. In a transmission hologram structure 40 shown in FIG. 5, low refractive index resin layers 41 and high refractive index resin layers 42 are alternately arranged along a laser beam incident direction (a direction of an arrow). A reflection hologram is formed by interference between an incident laser beam and specular reflection light. Therefore, as shown in FIG. 5, a reflection hologram structure 50, in which the low refractive index resin layers 41 and the high refractive index resin layers 42 are alternately arranged along a direction different from (for example, perpendicular to) the laser beam incident direction (the direction of the arrow), is formed. When the reflection hologram structure as shown in FIG. 5 is formed, in a case where the laser beam is visible light, this reflection hologram may cause a phenomenon in which a light diffusion film exhibits color. Therefore, when a light diffusion film is used in a liquid crystal display or the like, although an improved viewing angle is obtained, display properties may be deteriorated because of color due to a reflection hologram.